<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<link rel="STYLESHEET" href="lib.css" type='text/css' />
<link rel="SHORTCUT ICON" href="../icons/pyfav.png" type="image/png" />
<link rel='start' href='../index.html' title='Python documentation Index' />
<link rel="first" href="lib.html" title='Python library Reference' />
<link rel='contents' href='contents.html' title="Contents" />
<link rel='index' href='genindex.html' title='Index' />
<link rel='last' href='about.html' title='About this document...' />
<link rel='help' href='about.html' title='About this document...' />
<link rel="prev" href="debugger-commands.html" />
<link rel="parent" href="module-pdb.html" />
<link rel="next" href="profile.html" />
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name='aesop' content='information' />
<title>24.2 How It Works </title>
</head>
<body>
<div class="navigation">
<div id='top-navigation-panel' xml:id='top-navigation-panel'>
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="24.1 debugger Commands"
  href="debugger-commands.html"><img src='../icons/previous.png'
  border='0' height='32'  alt='Previous Page' width='32' /></a></td>
<td class='online-navigation'><a rel="parent" title="24. the Python Debugger"
  href="module-pdb.html"><img src='../icons/up.png'
  border='0' height='32'  alt='Up one Level' width='32' /></a></td>
<td class='online-navigation'><a rel="next" title="25. the Python Profilers"
  href="profile.html"><img src='../icons/next.png'
  border='0' height='32'  alt='Next Page' width='32' /></a></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
  href="contents.html"><img src='../icons/contents.png'
  border='0' height='32'  alt='Contents' width='32' /></a></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
  border='0' height='32'  alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
  href="genindex.html"><img src='../icons/index.png'
  border='0' height='32'  alt='Index' width='32' /></a></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="debugger-commands.html">24.1 Debugger Commands</a>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="module-pdb.html">24. The Python Debugger</a>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="profile.html">25. The Python Profilers</a>
</div>
<hr /></div>
</div>
<!--End of Navigation Panel-->

<h1><a name="SECTION0026200000000000000000"></a><a name="debugger-hooks"></a>
<br>
24.2 How It Works 
</h1>

<p>
Some changes were made to the interpreter:

<p>

<ul>
<li><code>sys.settrace(<var>func</var>)</code> sets the global trace function
</li>
<li>there can also a local trace function (see later)
</li>
</ul>

<p>
Trace functions have three arguments: <var>frame</var>, <var>event</var>, and
<var>arg</var>. <var>frame</var> is the current stack frame.  <var>event</var> is a
string: <code>'call'</code>, <code>'line'</code>, <code>'return'</code>, <code>'exception'</code>,
 <code>'c_call'</code>, <code>'c_return'</code>, or <code>'c_exception'</code>. <var>arg</var>
 depends on the event type.

<p>
The global trace function is invoked (with <var>event</var> set to
<code>'call'</code>) whenever a new local scope is entered; it should return
a reference to the local trace function to be used that scope, or
<code>None</code> if the scope shouldn't be traced.

<p>
The local trace function should return a reference to itself (or to
another function for further tracing in that scope), or <code>None</code> to
turn off tracing in that scope.

<p>
Instance methods are accepted (and very useful!) as trace functions.

<p>
The events have the following meaning:

<p>
<dl>
<dt><strong><code>'call'</code></strong></dt>
<dd>A function is called (or some other code block entered).  The global
trace function is called; <var>arg</var> is <code>None</code>;
the return value specifies the local trace function.

<p>
</dd>
<dt><strong><code>'line'</code></strong></dt>
<dd>The interpreter is about to execute a new line of code (sometimes
multiple line events on one line exist).  The local trace function is
called; <var>arg</var> is <code>None</code>; the return value specifies the new
local trace function.

<p>
</dd>
<dt><strong><code>'return'</code></strong></dt>
<dd>A function (or other code block) is about to return.  The local trace
function is called; <var>arg</var> is the value that will be returned.  The
trace function's return value is ignored.

<p>
</dd>
<dt><strong><code>'exception'</code></strong></dt>
<dd>An exception has occurred.  The local trace function is called;
<var>arg</var> is a triple <code>(<var>exception</var>, <var>value</var>,
<var>traceback</var>)</code>; the return value specifies the new local trace
function.

<p>
</dd>
<dt><strong><code>'c_call'</code></strong></dt>
<dd>A C function is about to be called.  This may be an extension function
or a builtin.  <var>arg</var> is the C function object.

<p>
</dd>
<dt><strong><code>'c_return'</code></strong></dt>
<dd>A C function has returned. <var>arg</var> is <code>None</code>.

<p>
</dd>
<dt><strong><code>'c_exception'</code></strong></dt>
<dd>A C function has thrown an exception.  <var>arg</var> is <code>None</code>.

<p>
</dd>
</dl>

<p>
Note that as an exception is propagated down the chain of callers, an
<code>'exception'</code> event is generated at each level.

<p>
For more information on code and frame objects, refer to the
<em class="citetitle"><a
 href="../ref/ref.html"
 title="Python Reference Manual"
 >Python Reference Manual</a></em>.

<p>

<div class="navigation">
<div class='online-navigation'>
<p></p><hr />
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="24.1 debugger Commands"
  href="debugger-commands.html"><img src='../icons/previous.png'
  border='0' height='32'  alt='Previous Page' width='32' /></a></td>
<td class='online-navigation'><a rel="parent" title="24. the Python Debugger"
  href="module-pdb.html"><img src='../icons/up.png'
  border='0' height='32'  alt='Up one Level' width='32' /></a></td>
<td class='online-navigation'><a rel="next" title="25. the Python Profilers"
  href="profile.html"><img src='../icons/next.png'
  border='0' height='32'  alt='Next Page' width='32' /></a></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
  href="contents.html"><img src='../icons/contents.png'
  border='0' height='32'  alt='Contents' width='32' /></a></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
  border='0' height='32'  alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
  href="genindex.html"><img src='../icons/index.png'
  border='0' height='32'  alt='Index' width='32' /></a></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="debugger-commands.html">24.1 Debugger Commands</a>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="module-pdb.html">24. The Python Debugger</a>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="profile.html">25. The Python Profilers</a>
</div>
</div>
<hr />
<span class="release-info">Release 2.5.1, documentation updated on 18th April, 2007.</span>
</div>
<!--End of Navigation Panel-->
<address>
See <i><a href="about.html">About this document...</a></i> for information on suggesting changes.
</address>
</body>
</html>
